Most of wireless communication terminals adopt and use an analog heterodyne reception scheme which down-converts RF signals into baseband signals or low Intermediate Frequency (IF) signals through a plurality of mixers and IF stages.
Since an existing reception structure using the heterodyne reception scheme uses a plurality of analog parts, a circuit construction is complicated and thus it is difficult to integrate the analog parts into one chip, and a volume increases. Furthermore, since consuming much power, the reception structure is not appropriate for a personal mobile communication device such as a Personal Digital Assistant (PDA) and a wireless terminal where miniaturization and mobility are important.
Therefore, a recently provided terminal adopts a reception structure based on direct-conversion scheme. Since the direct-conversion scheme performs only one time of frequency down conversion using one mixer, there is an advantage of minimizing an RF portion, and the direction-conversion scheme has a characteristic of being more flexible than the heterodyne scheme in an aspect of a hardware.
However, the direct-conversion scheme has a limitation of an I/Q unbalance generated by disagreement in phases and amplitudes between an I channel and a Q channel. The I/Q unbalance limitation is a factor deteriorating an entire receiver performance, and a circuit or signal processing for compensating for this I/Q unbalance is indispensably required. That is, a portion of RF signals is up-converted by the I/Q unbalance and moves to a baseband to cause interference to the down-converted signals. This interference increases as a degree of the I/Q unbalance increases, so that a received signal may become an irrecoverable state. Also, the I/Q unbalance greatly rotates constellation of received signals, thereby considerably deteriorating a Bit Error Rate (BER) performance after all.
As a most general method of compensating for the I/Q unbalance, there is a method in which when a test is performed after a terminal is manufactured, a test tone is generated in order to directly measure an I/Q unbalance, correction is made, and then the terminal is brought to the market. This method detects an image tone generated by an I/Q unbalance, and applies the detected image tone to an I/Q unbalance compensator. Also, examples of a method of compensating for an I/Q unbalance in a digital region include an adaptive signal processing method such as Least Means Square (LMS) and Recursive Least Square (RLS), and a signal processing method of Digital Signal Processing (DSP) such as a Symmetric Adaptive Decorrelation (SAD).
In an OFDM system, the above-described adaptive signal processing method is used, or a method of estimating an I/Q unbalance value from a plurality of OFDM symbols to perform compensation is used. However, such methods are not suitable for standards such as a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) and Mobile WiMAX. Also, in the method of generating the test tone, which is a square wave, inside a chip to compensate for an I/Q unbalance, when the test tone is generated during actual transmission/reception of data, collision between data and the test tone occurs, so that it is impossible to correct I/Q unbalance during actual transmission/reception of data. That is, according to this method, it is very difficult to adaptively take measures against an influence by an environment such as temperature change. Also, since the signal processing method of DSP that can perform correction in real-time includes repetition and convergence processes, a plurality of training symbols and much calculation time are required.
Besides these methods, a variety of methods for removing an I/Q unbalance from the OFDM scheme are being proposed, but they require the above-described convergence process, or require a specific pilot structure exclusively used for estimating an I/Q unbalance.